The rapid emergence of antibiotic resistance amongst pathogenic bacteria is a major clinical and public health problem. The established paradigm suggests that antibiotic resistance emerges by selecting for pre-existing mutants in the bacterial population exposed to antibiotics. However, recent data suggests that adaptive resistance mutations can occur in bacteria in response to antibiotic therapy [1, 2]. Adaptive resistance mutations may be caused by activation of the SOS DNA repair and mutagenesis pathway [3, 4]. The SOS response pathway is initiated by the accumulation of single-stranded DNA (ssDNA), promoting activation of RecA, inactivation of LexA repressor, and induction of SOS genes, including SOS error prone polymerases [5-7].
Bactericidal antibiotics are powerful instigators of the SOS response [1, 8]. Bactericidal antibiotics can induce a common mechanism of cell death by stimulating the formation of lethal amounts of oxidative radicals (hydroxyl radicals), which activates RecA and the SOS response [9]. E. coli strains lacking RecA are much more sensitive to bactericidal antibiotics or bacteriostatic antibiotics that are activators of the SOS response [9]. Thus, RecA can contribute to increased tolerance to antibiotic treatment by enhancing repair of DNA damage that occurs either directly by antibiotic-induced DNA damage or indirectly from metabolic and oxidative stress. RecA-mediated repair can also induce a hypermutable state that can promote acquisition of antibiotic resistance. If DNA damage is not successfully repaired, then mutagenic polymerases (PolIV and PolV) are induced, causing mutagenesis to occur and enabling bacteria to develop antibiotic resistance [1]. Bacteria can also develop antibiotic resistance by obtaining resistant genes from foreign DNA using the SOS response-mediated horizontal gene transfer pathway [10, 11].
SOS response mechanisms have also been confirmed in Gram-positive species [12]. RecA proteins, generally 318 to 388 amino acid residues in size [13], are nearly ubiquitous in bacterial species. RecA genes within protobacteria (including Gram-negative pathogens) and Gram-positive species are highly conserved [14]. Therefore, inhibitors of RecA can be used as broad-spectrum co-drugs against Gram-negative or Gram-positive pathogens.
A crystal structure of the post-ATP hydrolysis conformation of the archaebacteria Methanococcus voltae RecA homologue MvRadA was obtained by co-crystallizing it in the presence of ADP and the phosphate analog sodium tungstate (Na2WO4) [15]. A cluster of 12 tungsten atoms was located by outstanding anomalous scattering signals near DNA binding loops L1 and L2. The metatungstate (W12O406−) compound inhibited MvRadA ATPase, DNA-binding, and DNA strand-exchange activities [15]. This study showed that drug-sized molecules can competitively block DNA-binding by RecA-like protein filaments. However, metatungstate is unable to inhibit RecA activity within E. coli cells.
There remains a need for molecules that can limit the development of resistance to antibiotics.